Magnetoresistive element and magnetoresistive random access memory including the same

ABSTRACT

The present invention provides a low-resistance magnetoresistive element of a spin-injection write type. A crystallization promoting layer that promotes crystallization is formed in contact with an interfacial magnetic layer having an amorphous structure, so that crystallization is promoted from the side of a tunnel barrier layer, and the interface between the tunnel barrier layer and the interfacial magnetic layer is adjusted. With this arrangement, it is possible to form a magnetoresistive element that has a low resistance so as to obtain a desired current value, and has a high TMR ratio.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 12/211,388, filedSep. 16, 2008, which claims priority under U.S.C. 119 to JapaneseApplication No. 2007-248251, filed Sep. 25, 2007, the entire contents ofboth of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetoresistive element and amagnetoresistive random access memory including the magnetoresistiveelement.

2. Related Art

In recent years, a number of solid-state memories that recordinformation have been suggested on the basis of novel principles. Amongthose solid-state memories, magnetoresistive random access memories(hereinafter also referred to as MRAMs) that take advantage of tunnelingmagneto resistance (hereinafter also referred to as TMR) have been knownas solid-state magnetic memories. Each MRAM includes magnetoresistiveelements (hereinafter also referred to as MR elements) that exhibitmagnetoresistive effects as the memory elements of memory cells, and thememory cells store information in accordance with the magnetizationstates of the MR elements.

Each MR element includes a magnetization free layer having amagnetization where a magnetization direction is variable, and amagnetization reference layer having a magnetization of which adirection is invariable. When the magnetization direction of themagnetization free layer is parallel to the magnetization direction ofthe magnetization reference layer, the MR element is put into a lowresistance state. When the magnetization direction of the magnetizationfree layer is antiparallel to the magnetization direction of themagnetization reference layer, the MR element is put into a highresistance state. The difference in resistance is used in storinginformation.

As a method of writing information on such a MR element, a so-calledcurrent-field write method has been known. By this method, a line isplaced in the vicinity of the MR element, and the magnetization of themagnetization free layer of the MR element is reversed by the magneticfield generated by the current flowing through the line. When the sizeof the MR element is reduced to form a small-sized MRAM, the coerciveforce Hc of the magnetization free layer of the MR element becomeslarger. Therefore, in a MRAM of the current-field write type, thecurrent required for writing tends to be larger, since the MRAM issmall-sized. As a result, it is difficult to use a low current andsmall-sized memory cells designed to have capacity larger than 256Mbits.

As a write method designed to overcome the above problem, a write methodthat utilizes spin momentum transfers (SMT) (a spin-injection writingmethod or spin-transfer-torque writing method) has been suggested (seeU.S. Pat. No. 6,256,223). By the spin-injection write method, a currentis applied in a direction perpendicular to the film plane of each of thefilms forming a MR element having a tunneling magnetoresistive effect,so as to change (reverse) the magnetization state of the MR element.

In a magnetization reversal caused by spin injection, the current Icrequired for the magnetization reversal is determined by the currentdensity Jc. Accordingly, as the area of the face on which the currentflows becomes smaller in a MR element, the injection current Ic requiredfor reversing the magnetization becomes smaller. In a case where writingis performed with fixed current density, the current Ic becomes smaller,as the size of the MR element becomes smaller. Accordingly, thespin-injection writing method provides excellent scalability inprinciple, compared with the current-induced magnetic field writingmethod.

In a MRAM of a spin injection type, however, the current that can beapplied at the time of writing is determined by the voltage generated ata selective transistor and the relationship in resistance between theselective transistor and each TMR element. Therefore, it is necessary tolower the resistance of each TMR element, or to lower the resistance ofeach TMR film.

SUMMARY OF THE INVENTION

The present invention has been made in view of these circumstances, andan object thereof is to provide a magnetoresistive element of alow-resistance spin-injection writing type that allows the magnetizationfree layer to have a magnetization reversal with a low current, and amagnetoresistive random access memory that includes the magnetoresistiveelement.

A magnetoresistive element according to a first aspect of the presentinvention includes: a magnetization reference layer having magnetizationperpendicular to a film plane, a direction of the magnetization beinginvariable in one direction; a magnetization free layer havingmagnetization perpendicular to the film plane, a direction of themagnetization being variable; and an intermediate layer provided betweenthe magnetization reference layer and the magnetization free layer, atleast one of the magnetization reference layer and the magnetizationfree layer including: an interfacial magnetic layer formed in contactwith the intermediate layer, and having a crystalline phase crystallizedfrom an amorphous structure; and a crystallization promoting layerformed in contact with the interfacial magnetic layer on the oppositeside from the intermediate layer, and promoting crystallization of theinterfacial magnetic layer, the magnetization direction of themagnetization free layer being variable by flowing a current between themagnetization reference layer and the magnetization free layer via theintermediate layer.

A magnetoresistive element according to a second aspect of the presentinvention includes: a magnetization reference layer having magnetizationparallel to a film plane, a direction of the magnetization beinginvariable in one direction; a magnetization free layer havingmagnetization parallel to the film plane, a direction of themagnetization being variable; and an intermediate layer provided betweenthe magnetization reference layer and the magnetization free layer, atleast one of the magnetization reference layer and the magnetizationfree layer including: an interfacial magnetic layer formed in contactwith the intermediate layer, and having a crystalline phase crystallizedfrom an amorphous structure; and a crystallization promoting layerformed in contact with the interfacial magnetic layer on the oppositeside from the intermediate layer, and promoting crystallization of theinterfacial magnetic layer, the magnetization direction of themagnetization free layer being variable by flowing a current between themagnetization reference layer and the magnetization free layer via theintermediate layer.

A magnetoresistive random access memory according to a third aspect ofthe present invention includes: the magnetoresistive element accordingto any one of the first and second aspects as a memory cell.

A magnetoresistive random access memory according to a fourth aspect ofthe present invention includes: a memory cell that includes themagnetoresistive element according to any one of the first and secondaspects, and a transistor having one end series-connected to one end ofthe magnetoresistive element; a first write current circuit connected tothe other end of the magnetoresistive element; and a second writecurrent circuit connected to the other end of the transistor, and, incooperation with the first write current circuit, flowing the currentbetween the magnetization reference layer and the magnetization freelayer via the intermediate layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a TMR element in accordance with afirst embodiment of the present invention;

FIG. 2 is a cross-sectional view of a TMR element in accordance with afirst modification of the first embodiment;

FIG. 3 is a cross-sectional view of a TMR element in accordance with asecond modification of the first embodiment;

FIG. 4 is a cross-sectional view of a TMR element in accordance with afourth modification of the first embodiment;

FIG. 5 is a cross-sectional view of a TMR element in accordance with anexample 1;

FIG. 6 is a cross-sectional view of a TMR element in accordance with anexample 2;

FIG. 7 is a cross-sectional view of a TMR element in accordance with anexample 3;

FIG. 8 is a cross-sectional view of a TMR element in accordance with anexample 6;

FIG. 9 is a cross-sectional view of a TMR element in accordance with anexample 7;

FIG. 10 is a cross-sectional view of a memory cell in a MRAM inaccordance with a second embodiment; and

FIG. 11 is a circuit diagram for showing the principle components of theMRAM of the second embodiment.

DESCRIPTION OF THE EMBODIMENTS

The following is a description of embodiments of the present invention,with reference to the accompanying drawings. In the followingdescription, like components having like functions and structures aredenoted by like reference numerals, and explanation is repeated onlywhere necessary.

First Embodiment

FIG. 1 shows a magnetoresistive element (hereinafter also referred to asa TMR element) in accordance with a first embodiment of the presentinvention. The TMR element 1 of this embodiment includes: amagnetization reference layer 2 that includes a magnetic film having amagnetization of which a direction is invariable in one direction; amagnetization free layer 6 that includes a magnetic film having amagnetization of which a direction is variable; and an intermediatelayer 4 that is provided between the magnetization reference layer 2 andthe magnetization free layer 6.

In general, a TMR element is an element that is designed to be in one oftwo steady states in accordance with the direction of the bidirectionalcurrent flowing in a direction perpendicular to the film plane. The twosteady states are associated with “0” date and “1” data, respectively,so that the TMR element can store binary data. This is called thespin-injection writing type (or spin-transfer-torque writing type), bywhich the magnetization is varied with the direction of the currentflowing direction, and information corresponding to the magnetizationstate is stored. When there is the “0” data, the magnetizationdirections of the magnetization free layer 6 and the magnetizationreference layer 2 are in a parallel state. When there is the “1” data,the magnetization directions are in an antiparallel state. Themagnetization directions of the magnetization free layer 6 and themagnetization reference layer 2 are substantially parallel to the filmplane, or are substantially perpendicular to the film plane. Themagnetization substantially parallel to the film plane will behereinafter also referred to as the in-plane magnetization, and themagnetization substantially perpendicular to the film plane will behereinafter also referred to as the perpendicular magnetization.

In this embodiment, the magnetization reference layer 2 includes aninterfacial magnetic layer 2 a, a crystallization promoting layer 2 b,and an assisting magnetic film 2 c. The interfacial magnetic layer 2 ais provided on the opposite side of the intermediate layer 4 from themagnetization free layer 6. The assisting magnetic film 2 c is providedon the opposite side of the interfacial magnetic layer 2 a from theintermediate layer 4. The crystallization promoting layer 2 b isprovided between the interfacial magnetic layer 2 a and the assistingmagnetic film 2 c.

Also, the magnetization free layer 6 includes an interfacial magneticlayer 6 a, a crystallization promoting layer 6 b, and an assistingmagnetic film 6 c. The interfacial magnetic layer 6 a is provided on theopposite side of the intermediate layer 4 from the magnetizationreference layer 2. The assisting magnetic layer 6 c is provided on theopposite side of the interfacial magnetic layer 6 a from theintermediate layer 4.

The crystallization promoting layer 6 b is provided between theinterfacial magnetic layer 6 a and the assisting magnetic layer 6 c.

(First Modification)

FIG. 2 shows a TMR element in accordance with a first modification ofthis embodiment. The TMR element 1A of the first modification differsfrom the TMR element 1 of the first embodiment shown in FIG. 1 in thatthe magnetization free layer 6 is a single-layer interfacial magneticlayer without the crystallization promoting layer 6 b and the assistingmagnetic layer 6 c, and a crystallization promoting layer 8 is providedon the opposite side of the magnetization free layer 6 from theintermediate layer 4.

(Second Modification)

FIG. 3 shows a TMR element in accordance with a second modification ofthis embodiment. The TMR element 1B of the second modification differsfrom the TMR element 1 of the first embodiment shown in FIG. 1 in thatthe magnetization reference layer 2 is a single-layer interfacialmagnetic layer without the crystallization promoting layer 2 b and theassisting magnetic layer 2 c, and a crystallization promoting layer 10is provided on the opposite side of the magnetization reference layer 2from the intermediate layer 4.

(Third Modification)

FIG. 4 shows a TMR element in accordance with a third modification ofthis embodiment. The TMR element 1C of the third modification differsfrom the TMR element 1A of the first modification shown in FIG. 2 inthat the magnetization reference layer 2 is a single-layer interfacialmagnetic layer without the crystallization promoting layer 2 b and theassisting magnetic layer 2 c, and a crystallization promoting layer 10is provided on the opposite side of the magnetization reference layer 2from the intermediate layer 4.

As described above, in each of the TMR elements in accordance with thisembodiment and its modifications, a stacked structure formed with anintermediate layer, an interfacial magnetic layer, and a crystallizationpromoting layer (a crystallization promoting layer) is provided at bothsides of the intermediate layer 4. However, the stacked structure may beprovided only on one side of the intermediate layer 4.

Further, in this embodiment, the first modification, and the secondmodification, either the magnetization reference layer 2 or themagnetization free layer 6 has an assisting magnetic layer. Thisassisting magnetic layer is designed to generate anisotropy energy, whenassisting the perpendicular magnetization of either the magnetizationreference layer 2 or the magnetization free layer 6, or increasing theresistance to thermal disturbance.

In this embodiment and its modifications, the crystallization promotinglayers 8 and 10 and the crystallization promoting layers 2 b and 6 bhave an “effect of facilitating crystallization of an interfacialmagnetic layer”. When crystallization of an interfacial magnetic layeris started from the interface with the intermediate layer 4, the latticemismatching at the interface is smaller, and lower resistance and ahigher TMR ratio can be expected. As incidental effects, unnecessaryoxygen is absorbed from the interfacial magnetic layer, andcrystallization is facilitated.

In a case where a tunnel barrier layer made of an insulating oxidematerial (hereinafter also referred to as the barrier layer) is used asthe intermediate layer 4, the crystallization promoting layers and thecrystallization promoting layers have the effect of absorbing excessiveoxygen from the barrier layer to make the barrier layer similar to thestoichiometric composition and to prevent the barrier layer from beingin a peroxidative state. This interfacial magnetic layer mainly has anamorphous structure immediately after its formation. Here, the “mainly”means that the amorphous structure occupies 50% or more area whenobserved in the film plane, or 50% or more of the volume in the filmplane is the amorphous structure. An “amorphous structure” does not havelong-range order like crystals, but has short-range order. Because ofthe crystalline structure, only the first-nearest atoms are defined. Thenumber of the first-nearest atoms and the types of atoms can be analyzedby a technique such as the EXAFS (Extended X-ray Adsorption FineStructure) technique. Also, the embodiment of the present inventioncontains a polycrystalline film having mean crystal grains of 2 nm orsmaller in diameter. This is because chances are that the structurecannot be determined whether to be a crystalline structure or anamorphous structure. Although the interfacial magnetic layer in theembodiment of the present invention is an amorphous structureimmediately after the film formation, the interfacial magnetic layer ischaracteristically crystallized from the amorphous structure byinjecting excitation energy generated by carrying out heat treatmentimmediately after the film formation, generating Joule heat a current,or performing ion irradiation.

In this embodiment and its modifications, the intermediate layer 4 is atunnel barrier layer (hereinafter also referred to as the barrierlayer). The barrier layer is made of an oxide having a NaCl structure.Specific examples of the materials that can be used for the barrierlayer include CaO, MgO, SrO, BaO, and TiO, which are oxides of Be, Ca,Mg, Sr, Ba, Ti, and the likes. Alternatively, the barrier layer may bemade of a mixed crystal material of those oxides. With the easiness offormation and the processability of the barrier layer being taken intoconsideration, MgO is practical and exhibits the highest MR ratio.

If the above described barrier layer of a NaCl structure has anepitaxially matched interface formed on the (001) plane with respect toa Fe_(1-x-y)Co_(x)Ni_(y) (0≦x≦1, 0≦y≦1, 0≦x+y≦1) magnetic film having aBCC (body-centered cubic) structure, a high TMR ratio can be achieved.If a high TMR ratio is achieved, the following preferred relationshipsshould be established between the (001) plane of the barrier layer andthe (001) plane of the magnetic film of a BCC structure:

[001] direction of barrier layer//[110] direction of magnetic film ofBCC structure

(100) plane of barrier layer//(100) plane of magnetic film of BCCstructure

Here, the symbol “//” means “being parallel”.

It is preferable that the lattice misfit at the interface is small, soas to maintain the above orientation and direction relationships.

Further, if good lattice matching is maintained at the interface, thebonding between the band structures of the magnetic film and the barrierlayer is good in an electronic state, and coherent electronic tunnelingoccurs. Ideally, when coherent electron tunneling occurs, the resistanceof the TMR element including the magnetization reference layer 2, thetunnel barrier layer 4, and the magnetization free layer 6 becomeslower, and a high TMR ratio can be expected. To cause coherent tunnelingin such a case, lattice matching is required at both interfaces of thebarrier layer of a NaCl structure.

In a case where the (100) plane of the barrier layer and the (110) planeof the magnetic film of a BCC structure form an interface, the productof resistance and area RA standardized by the area becomes 10 to 100times higher than the RA observed in a case of an matched interface, dueto an increase in interfacial lattice misfit dislocation. Here, Rrepresents the resistance of the element, and A represents the area ofthe element.

In a case where the barrier layer of a NaCl structure is grown directlyon the (100) plane of the above described magnetic film having a BCCstructure, it is difficult to reduce deformation of the MgO lattice, anda misfit dislocation occurs to lower the TMR ratio. This is becausemismatching is caused at the interface due to the misfit dislocation atthe interface.

For the reasons described above, it is very difficult to grow a barrierlayer with a NaCl structure orientated to the (100) plane on the (100)plane of a magnetic film of a BCC structure. In such a case, other thanthe (100) plane, a mixed phase state in which the (111) plane is mixedwith the (100) plane appears, and crystal grains orientated to the (100)plane and crystal grains orientated to the (111) plane exist at random.Accordingly, the energy increases caused by the interface misfit withthe (100) plane of the magnetic film serving as the under layer can bereduced. Thus, the misfit dislocation due to the lattice misfit at theinterface becomes larger, and the product of resistance and area RA ofthe TMR element becomes higher.

The barrier layer with a NaCl structure on the under layer having anamorphous structure, as described above, easily has crystal growthpreferentially orientated to the (100) plane. If the interfacialmagnetic layer in the embodiment of the present invention is to functionas the under layer to form a barrier layer of a TMR element, an optimummaterial is an alloy formed by adding a half-metal element such as B, P,S, or C, or N (nitrogen), or a semiconductor element such as Si, Ge, orGa to a Fe_(1-x-y)Co_(x)Ni_(y) (0≦x≦1, 0≦y≦1, 0≦x+y≦1) alloy inherentlyhaving a BCC structure. Each of those materials is crystallized throughan excitation process such as heat treatment, and a BCC structure phaseis then precipitated.

The optimum thickness of the interfacial magnetic layer is in the rangeof approximately 0.1 nm to 5 nm. If the thickness of the interfacialmagnetic layer is smaller than 0.1 nm, a high TMR ratio and a low RAcannot be achieved. If the thickness exceeds 5 nm, the interfacialmagnetic layer is much larger than the characteristic length with whicha spin torque can be applied, and the magnetization free layer cannothave a magnetization reversal through spin injection. Therefore, thethickness of the interfacial magnetic layer is optimized so as torestrict the film thickness of the magnetization free layer to 5 nm orless.

The interfacial magnetic layer of the embodiment of the presentinvention should preferably contain at least one element selected fromthe group consisting of Fe, Co, and Ni at 50 atomic % or more. This isbecause, if the interfacial magnetic layer contains the element at lessthan 50 atomic %, magnetization might disappears. In such a case, thereis a high probability that the polarizability of the interfacialmagnetic layer also becomes lower and disappears. Even if the resistancecan be lowered, a MR ratio cannot be measured.

To crystallize the above-mentioned amorphous phase, it is necessary tocarry out heat treatment. When a BCC structure is precipitated throughcrystallization, it is necessary to prepare the origin ofcrystallization.

In this embodiment and its modifications, the interfacial magnetic layerhaving an amorphous structure stabilizes the entire energy throughcrystallization originated from the interface with the intermediatelayer 4. More specifically, to achieve a high TMR ratio with a lowcurrent, it is necessary to facilitate crystallization of theinterfacial magnetic layer having an amorphous structure, starting fromthe interface side of the tunnel barrier layer 4. In the case wherecrystallization starts from the side of the tunnel barrier layer 4, itis considered that the crystallization progresses by minimizing themisfit energy at the interface with the tunnel barrier layer while anappropriate amount of an additional material such as B, P, S, C, or N iscontained. As a result, a very small amount of B, P, S, C, or N remainsin the phase of the magnetic film of a BCC structure afterrecrystallization. Thus, the misfit dislocations at the interface can berestricted to a low amount.

In the above described interfacial magnetic layer, crystallization froman amorphous phase occurs first at the interface with lower interfaceenergy. In this case, the crystallization structure and orientationafter the crystallization are determined so that the interface energy ofthe interface at which the crystallization starts is reduced.

In this embodiment and its modifications, the interfacial magnetic layerhaving an amorphous structure is in contact with the barrier layer 4 ofa NaCl structure on its (001) plane. Therefore, crystallization shouldideally start at the interface on the side of the barrier layer 4 of aNaCl structure. In this case, the interfacial magnetic layer inevitablyforms an interface epitaxially matching with the barrier layer 4 of aNaCl structure from an amorphous structure, and a BCC structure phasegrows from this interface, orientated to the (001) plane. Here, thecrystallization progresses while the following crystalline directionrelationships (also described above) are maintained:

[001] direction of barrier layer//[110] direction of magnetic film ofBCC structure

(100) plane of barrier layer//(100) plane of magnetic film of BCCstructure

The inventors discovered what kind of layer should be brought intocontact with the interface on the opposite side of the interfacialmagnetic layer (crystallizing from an amorphous phase) from the barrierlayer, so that the magnetic film having an amorphous structure iscrystallized from the barrier layer side as described above. In otherwords, to facilitate crystallization from the barrier layer side, alayer made of a material that is crystallized at a lower speed is formedon the side of the other interface.

Here, the entire magnetic layer having an amorphous structure does notneed to be crystallized, but the interface with the intermediate layershould be crystallized. The spin-injection magnetization reversalcurrent might become lower in a case where only the interface with theintermediate layer (the barrier layer) and its neighboring area arecrystallized.

More specifically, it is preferable to employ an element that forms aeutectic state with Fe, Co, and Ni, instead of a total solid-solublestate. It is more preferable to employ an element having a highermelting point than Fe, Co, and Ni. The melting points of Fe, Co, and Niare 1540° C., 1490° C., and 1450° C., respectively.

As for the crystalline structure of the magnetic film, it is preferableto use an element having a BCC structure or a hexagonal closed pack(HCP) structure, other than a face-closed cubic (FCC) structure.Alternatively, it is preferable to use a covalent bonding element.

Next, materials that can be used for the crystallization promotinglayers (or the crystallization promoting layers) in this embodiment andits modifications are described.

In each TMR element of this embodiment and its modifications, thecrystallization promoting layer (or the crystallization promoting layer)is made of a rare earth element selected from the group consisting ofCe, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu (hereinafter alsoreferred to as the element A). Those rare earth elements have almost nosolid solubility limits, when combined with Fe, Co, or Ni. Each magneticfilm in the magnetization free layers and the magnetization referencelayers of this embodiment and the first modification is made of an alloycontaining an element selected from the group consisting of Fe, Co, andNi, and is hardly solid soluble in a rare earth metal.

At the time of film formation by a sputtering technique or the like, thecrystallization promoting layer (the crystallization promoting layer)causes mixing with the interfacial magnetic layer in an amorphous stateat the interface, and partially forms an amorphous phase of Fe, Co, orNi and a rare earth metal. The amorphous phase of Fe, Ni, or Co and arare earth element has a high crystallization temperature. Accordingly,crystallization of the interfacial magnetic layer from an amorphousphase easily starts at the interface between the interfacial magneticlayer and the barrier layer.

Among rare earth elements, Gd is a ferromagnetic material havingspontaneous magnetization. Accordingly, the exchange coupling betweenthe interfacial magnetic layers 2 a and 6 a and the assisting magneticlayers 2 c and 6 c is not cut off by the addition of the crystallizationpromoting layers 2 b and 6 b. Also, in a case where Gd is employed,there are no upper limits set on the thicknesses of the crystallizationpromoting layers 2 b and 6 b, as long as they are in-plane magnetizationfilms. The lower limit of the film thicknesses is 0.1 nm. If thecrystallization promoting layers 2 b and 6 b are thinner than 0.1 nm,the insertion effect cannot be achieved. In a case where themagnetization free layer and the magnetization reference layer includingthe crystallization promoting layers have perpendicular magnetization,the thickness of the Gd layer should preferably be 2 nm or less, so asto maintain the perpendicular properties of the layer containing Gd.

Meanwhile, elements such as Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb,and Lu do not have magnetism as single metals. However, each of thoseelements is alloyed with an element selected from the group consistingof Fe, Co, and Ni, and forms an amorphous structure or an intermetalliccompound. In this manner, those elements come to have magnetismgenerated from the orbital moment. Likewise, Gd is alloyed with anelement selected from the group consisting of Fe, Co, and Ni, and formsan amorphous structure or an intermetallic compound. In this manner, Gdcomes to have magnetism generated from the orbital moment.

In a case where a high-energy film formation process involving asputtering technique or the like is carried out on a single metal madeof a rare metal element selected from the group consisting of Ce, Pr,Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu, an amorphous phase is formedwhen mixing with the magnetic film serving as the under layer is caused.Likewise, a mixing layer is formed on the rare-earth single-metal layer,and an amorphous phase is formed. Accordingly, the magnetic coupling orexchange coupling between the interfacial magnetic layers 2 a and 6 aand the assisting magnetic layers 2 c and 6 c is not cut off. In thiscase, the film thickness of each crystallization promoting layer shouldbe 1 nm or less. This is because the mixing is caused approximately 0.5nm above or below the crystallization promoting layer.

The crystallization promoting layer (the crystallization promotinglayer) having magnetism and being exchange-coupled to the interfacialmagnetic layer or the assisting magnetic layer is essential in a casewhere the magnetization reference layer and the magnetization free layerhave perpendicular magnetization.

For example, in many cases, the magnetization reference layer 2 and theassisting magnetic layers 2 c and 6 c used in the magnetization freelayer 6 have perpendicular magnetization, and the interfacial magneticlayers 2 a and 6 a have in-plane magnetic films, as shown in FIG. 1. Tocause the interfacial magnetic layers 2 a and 6 a to have perpendicularmagnetization in this case, it is necessary to maintain the exchangecoupling between the crystallization promoting layers 2 b and 6 b andthe interfacial magnetic layers 2 a and 6 a, and the exchange couplingbetween the crystallization promoting layers 2 b and 6 b and thecrystallization promoting layers 2 c and 6 c.

When a rare earth element is alloyed with Co, Ni, or Fe (hereinafteralso referred to as the element X), an amorphous structure is formed.The definition of an amorphous structure has already been described. Analloy of an amorphous structure formed with a rare earth element and theelement X (Fe, Co, or Ni) can have perpendicular magnetization. Anamorphous alloy formed with a rare earth element and the element X is aferrimagnetic body. Accordingly, the amorphous alloy has thecompensation point composition with which the net saturationmagnetization Ms is zero. The compensation point composition isindicated by the atomic percentage (atomic %) of the rare earth element.If the amount of the rare earth element exceeds the compensation pointcomposition, the saturation magnetization Ms takes a negative value. Inother words, the field applying direction and the magnetizationdirection become opposite to each other. Since the saturationmagnetization Ms becomes smaller when the composition is close to thecompensation point, the effective magnetic crystalline anisotropy(K_(u-effect)) becomes larger, and stable perpendicular magnetizationcan be readily achieved. Accordingly, the above-described XA amorphousalloy is the optimum material for the crystallization promoting layer(the crystallization promoting layer) in a case of a MR element havingperpendicular magnetization. Examples of amorphous structure alloys thathave perpendicular magnetization and are formed with rare earth elementsand the element A include a TbCoFe alloy, a GdCoFe alloy, and a TbGdCoFealloy. It is also possible to add Ho or Dy to those alloys.

The crystallization promoting layer made of a rare earth element Aselected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, and Lu should preferably remain an amorphous structureeven after the interfacial magnetic layer is crystallized, except forthe case where the element A is Gd. The film thickness of thecrystallization promoting layer (the crystallization promoting layer)made of an amorphous alloy formed with a rare earth element and theelement X is not limited, and should preferably be in the range of 0.1nm to 10 nm.

In each TMR element of this embodiment and its modifications, thecrystallization promoting layer (the crystallization promoting layer)may be made of an element selected from the group consisting of Mg, Ca,Sc, Ti, Sr, Y, Zr, Nb, Mo, Ba, La, Hf, Ta, and W (hereinafter referredto as the element B). Each of those elements has a BCC structure or aHCP structure. In a case where the element B is an element selected fromthe group consisting of Mg, Ca, Sc, Ti, Zr, Y, and Sr, a HCP structureis formed. In a case where the element B is an element selected from thegroup consisting of Ta, W, Nb, Mo, Ba, Hf, and La, a BCC structure isformed.

The above-described elements have almost no solid solubility limits withrespect to Fe, Co, and Ni (hereinafter also referred to as the elementX), and each of the elements can form an intermetallic compound. Each ofthose elements easily forms an amorphous structure phase, when alloyed.As will be described later in detail, those elements exhibit almost nosolid solubility limits with a noble metal element (hereinafter referredto as the element Y) such as Pt, Pd, Au, Ag, Ru, Rh, Ir, or Os, oralloys.

The above-mentioned elements, Mg, Ca, Sc, Ti, Sr, Y, Zr, Nb, Mo, Ba, La,Hf, Ta, and W, are nonmagnetic elements. Therefore, the thickness of thecrystallization promoting layer is restricted to 1 nm or less. If thethickness exceeds 1 nm, the exchange coupling between the interfacialmagnetic layers 2 a and 6 a and the assisting magnetic layers 2 c and 6c is cut off. The crystallization promoting layer (the crystallizationpromoting layers) that has a thickness of 1 nm or less and is made of anelement (hereinafter referred to as the element B) selected from thegroup consisting of Mg, Ca, Sc, Ti, Sr, Y, Zr, Nb, Mo, Ba, La, Hf, Ta,and W causes mixing with the layer serving as the under layer, and formsan amorphous structure. Furthermore, the element B has a great effect ofattracting oxygen atoms contained in the interfacial magnetic layers,because the element B has higher electronegativity than the element Xsuch as Fe, Co, or Ni, and easily attracts oxygen. In each interfacialmagnetic layer in this case, more oxygen is observed at locations closerto the crystallization promoting layer (the crystallization promotinglayer). Accordingly, the oxygen concentration on the interface side ofthe barrier layer is lower, and crystallization from an amorphousstructure is facilitated.

The crystallization promoting layer (the crystallization promotinglayer) made of the element B may be crystallized to form a BCC structureor a HCP structure. In the case of a BCC structure, the orientation tothe (100) plane is observed. In the case of a HCP structure, orientationdoes not matter.

In each TMR element of this embodiment and its modifications, thecrystallization promoting layer may be made of an element (hereinafteralso referred to as the element C) selected from the group consisting ofSi, Ge, and Ga. Those elements are semiconductor elements havingcovalent bonding. The above-described elements have almost no solidsolubility limits with respect to Fe, Ci, and Ni, and easily formamorphous structures when alloyed. As will be described later in detail,those elements have almost no solid solubility limits with respect to anoble metal element (the element Y) such as Pt, Pd, Au, Ag, Ru, Rh, Ir,or Os, or an alloy. Therefore, each assisting magnetic layer shouldpreferably be an alloy or a stacked structure containing at least oneelement selected from the group consisting of Fe, Co, and Ni, and atleast one element selected from the group consisting of Ru, Rh, Pd, Ag,Os, Ir, Pt, and Au.

The assisting magnetic layers used in the embodiment of the presentinvention are now described. Each of those assisting magnetic layers hasthe function of assisting and reinforcing the perpendicularmagnetization characteristics of the magnetization reference layer orthe magnetization free layer, and also has the function of improving theheat disturbance resistance of the magnetization reference layer or themagnetization free layer. These assisting magnetic layers are designedto provide magnetic crystalline anisotropy energy. Accordingly, in acase where the interfacial magnetic layer has sufficient perpendicularmagnetization characteristics and heat disturbance resistance, it is notnecessary to employ the assisting magnetic layers. The thickness of theassisting magnetic layer used in the magnetization free layer should be5 nm or smaller, so that spin-injection magnetization reversals can becaused. A thickness of 5 nm or more is much larger than thecharacteristic length with which a spin torque is validly applied, andthe magnetization free layer cannot have a magnetization reversalthrough spin injection if the assisting magnetic layer is thicker than 5nm. The assisting magnetic layer used in the magnetization referencelayer should have such a thickness as not to have a reversal when themagnetization free layer has a magnetization reversal. Therefore, thefollowing relationship should be established:

M _(s-free) ·t _(free) <M _(s-reference) ·t _(reference)

where M_(s-free), M_(s-reference), t_(free), and t_(reference) representthe saturation magnetization of the magnetization free layer, thesaturation magnetization of the magnetization reference layer, the filmthickness of the magnetization free layer, and the film thickness of themagnetization reference layer, respectively.

The assisting magnetic layers having perpendicular magnetization are nowdescribed. Here, “perpendicular magnetization” and “magnetizationsubstantially perpendicular to the film plane” are defined as the statein which the ratio (Mr/Ms) between the residual magnetization Mr and thesaturation magnetization Ms when there is not a magnetic field is 0.5 orhigher in the magnetization-field (M-H) curve obtained by carrying outVSM (vibration sample magnetization) measurement. The characteristiclength with which a spin torque is validly applied is approximately 1.0nm. Examples of the materials that exhibit perpendicular magnetizationinclude a CoPt alloy, a CoCrPt alloy, and a CoCrPtTa alloy that havehexagonal closed pack (HCP) structures or face-centered cubic (FCC)structures. To exhibit magnetization perpendicular to the film plane,the material needs to be orientated to the (001) plane in a HCPstructure, and needs to be orientated to the (111) plane in a FCCstructure. A phase transition layer having a CsCl ordered structurephase tends to be orientated to the (110) plane.

The examples of the materials that exhibit perpendicular magnetizationalso include a RE-TM alloy that is formed with a rare earth metal(hereinafter also referred to as a RE) and an element selected from thegroup consisting of Co, Fe, and Ni (hereinafter also referred to as theTM element), and has an amorphous structure. The net saturationmagnetization of the RE-TM alloy can be controlled to switch from anegative value to a positive value by adjusting the amount of the REelement. The point where the net saturation magnetization Ms-net becomeszero is called the compensation point, and the composition observed atthat point is called the compensation point composition. In thecompensation point composition, the proportion of the RE element fallsin the range of 25 atomic % to 50 atomic %.

The examples of the materials that exhibit perpendicular magnetizationalso include an artificial-lattice perpendicular magnetization filmformed with multilayer stacked layers: a magnetic layer containing anelement selected from the group consisting of Co, Fe, and Ni; and anonmagnetic metal layer containing Pd, Pt, Au, Rh, Ir, Os, Ru, Ag, orCu. The material of the magnetic layer may be a Co_(100-x-y)Fe_(x)Ni_(y)alloy film (0≦x≦100, 0≦y≦100). It is also possible to employ a CoFeNiBamorphous alloy having B added to the above CoFeNi alloy at 10 to 25atomic %. The optimum film thickness of the magnetic layer is in therange of 0.1 nm to 1 nm. The optimum thickness of the nonmagnetic layeris in the range of 0.1 nm to 3 nm. The crystalline structure of theartificial lattice film may be a HCP structure, a FCC structure, or aBCC structure. In the case of a FCC structure, the artificial latticefilm is partially orientated to the (111) plane. In the case of a BCCstructure, the artificial lattice film is partially orientated to the(110) plane. In the case of a HCP structure, the artificial lattice filmis partially orientated to the (001) plane. The orientation can beobserved through X-ray diffraction or electron beam diffraction.

The examples of the materials that exhibit perpendicular magnetizationalso include a FCT ferromagnetic alloy that has a L1₀ ordered structureand is formed with at least one element selected from the groupconsisting of Fe and Co (hereinafter referred to as the element A), andat least one element selected from the group consisting of Pt and Pd(hereinafter referred to as the element B). Typical examples of L1₀ordered structure ferromagnetic alloys include an L1₀-FePt alloy, anL1₀-FePd alloy, and an L1₀-CoPt alloy. It is also possible to employ anL1₀-FeCoPtPd alloy that is an alloy formed with the above alloys. Toform such a L1₀ ordered structure, “x” needs to be in the range of 30atomic % to 70 atomic %, where the relative proportions of the element Aand the element B are expressed as A_(100-x)B_(x). Part of the element Acan be replaced with Ni or Cu. Part of the element B can be replacedwith Au, Ag, Ru, Rh, Ir, Os, or a rare earth element (such as Nd, Sm,Gd, or Tb). In this manner, the saturation magnetization Ms and themagnetic crystalline anisotropy energy (uniaxial magnetic anisotropyenergy) K_(u) of the magnetization free layer having perpendicularmagnetization can be adjusted and optimized.

The above-described ferromagnetic AB alloy having a L1₀ orderedstructure is a face-centered tetragonal (FCT) structure. By regulatingthe structure, large magnetic crystalline anisotropy energy ofapproximately 1×10⁷ erg/cc can be obtained in the [001] direction. Inother words, excellent perpendicular magnetization characteristics canbe achieved through preferential orientation toward the (001) plane. Thesaturation magnetization is approximately in the range of 600 emu/cm³ to1200 emu/cm³. In a case where an element is added to the alloy byreplacing a component with the element A or the element B, thesaturation magnetization and the magnetic crystalline anisotropy energybecome smaller. On the (001) plane of the ferromagnetic AB alloy havingthe above described L1₀ ordered structure, a BCC structure alloycontaining Fe, Cr, V, or the like as a principal component easily grows,preferentially orientated to the (001) plane.

The preferential orientation of a FCT-FePt alloy to the (001) plane canbe observed as a (002) peak in the neighborhood of the point where 2θ is45 to 50 degrees by performing a θ-2θ scan with X-ray diffraction. Toimprove the perpendicular magnetization characteristics, the half widthof the rocking curve of the (002) diffraction peak needs to be 10degrees or less, and, more preferably, 5 degrees or less.

The existence of a L1₀ ordered structure phase and the preferentialorientation to the (001) plane can be observed as a (001) diffractionpeak in the neighborhood of the point where 20 is 20 to 25 degrees byperforming a θ-2θ scan with X-ray diffraction.

Those diffraction images that are derived from the (001) plane and the(002) plane can be observed through electron beam diffraction or thelike.

EXAMPLES

Next, specific stacked structures of TMR elements are described asexamples in accordance with the present invention in detail.

Example 1

FIG. 5 shows a TMR element of a coercitivity differential type asExample 1 in accordance with the present invention. The TMR element ofExample 1 is of a bottom-reference type. More specifically, amagnetization reference layer 2 is formed on a under layer 12, anintermediate layer 4 is formed on the magnetization reference layer 2, amagnetization free layer 6 is formed on the intermediate layer 4, and acap layer 14 is formed on the magnetization free layer 6. Themagnetization reference layer 2 is a stacked structure that includes anassisting magnetic layer 2 c formed on the under layer 12, acrystallization promoting layer 2 b formed on the assisting magneticlayer 2 c, and an interfacial magnetic layer 2 a formed on thecrystallization promoting layer 2 b. In this example, the magneticreference layer 2 and the magnetization free layer 6 may both havemagnetization perpendicular to the film plane, or may both havemagnetization parallel to the film plane.

Example 2

FIG. 6 shows a TMR element of a coercitivity differential type asExample 2 in accordance with the present invention. The TMR element ofExample 2 is of a bottom-reference type, and is the same as the TMRelement of Example 1 shown in FIG. 5, except that a crystallizationpromoting layer 8 is provided between the magnetization free layer 6 andthe cap layer 14. In this example, the interfacial magnetic layerserving as the magnetization free layer 6 may not be made of a materialthat is crystallized from an amorphous structure, but may be made of amagnetic material that is crystallized in the first place. In this case,the crystallization promoting layer 8 plays a role of an excess oxygenabsorbing layer to adjust the stoichiometric composition of the barrierlayer (the intermediate layer) 4. In this example, the magneticreference layer 2 and the magnetization free layer 6 may both havemagnetization perpendicular to the film plane, or may both havemagnetization parallel to the film plane.

Example 3

FIG. 7 shows a TMR element of a coercitivity differential type asExample 3 in accordance with the present invention. The TMR element ofExample 3 is of a top-reference type. More specifically, a magnetizationfree layer 6 is formed on a under layer 12, an intermediate layer 4 isformed on the magnetization free layer 6, a magnetization referencelayer 2 is formed on the intermediate layer 4, and a cap layer 14 isformed on the magnetization reference layer 2. The magnetization freelayer 6 is a stacked structure that includes an assisting magnetic layer6 c formed on the under layer 12, a crystallization promoting layer 6 bformed on the assisting magnetic layer 6 c, and an interfacial magneticlayer 6 a formed on the crystallization promoting layer 6 b. Themagnetization reference layer 2 is a stacked structure that includes aninterfacial magnetic layer 2 a formed on the intermediate layer 4, acrystallization promoting layer 2 b formed on the interfacial magneticlayer 2 a, and an assisting magnetic layer 2 c formed on thecrystallization promoting layer 2 b. In this example, the magneticreference layer 2 and the magnetization free layer 6 may both havemagnetization perpendicular to the film plane, or may both havemagnetization parallel to the film plane.

Specific example structures of the respective TMR elements of Examples 1to 3 are shown below. The numeric values shown in the brackets indicatefilm thicknesses.

Specific Example Structure of Example 1

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (1 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/crystallization promoting layer 2 b made of Ta(0.2 nm)/assisting magnetic layer 2 c made of FePt (10 nm)/under layer12/thermally oxidized Si substrate (not shown)

Specific Example Structure of Example 2

Cap layer 14/crystallization promoting layer 8 made of Mg (0.5nm)/magnetization free layer 6 made of FePt (3 nm)/intermediate layer(barrier layer) 4 made of MgO (1 nm)/interfacial magnetic layer 2 a madeof CoFeB (2 nm)/crystallization promoting layer 2 b made of Ta (0.2nm)/assisting magnetic layer 2 c made of FePt (10 nm)/under layer12/thermally oxidized Si substrate (not shown)

Specific Example Structure of Example 3

Cap layer 14/assisting magnetic layer 2 c made of FePt (10nm)/crystallization promoting layer 2 b made of Mg (0.5 nm)/interfacialmagnetic layer 2 a made of Fe (1.5 nm)/intermediate layer (barrierlayer) 4 made of MgO (0.7 nm)/interfacial magnetic layer 6 a made ofCoFeB (0.5 nm)/crystallization promoting layer 6 b made of Ta (0.2nm)/assisting magnetic layer 6 c made of FePt (2 nm)/under layer12/thermally oxidized Si substrate (not shown)

The above described specific example structures are TMR elements havingperpendicular magnetization. In the case of a TMR element that has theperpendicular magnetization shown in each specific example structure ofExamples 1 and 2, a FePt film having a L1₀ ordered structure is employedas the interfacial magnetic layer 6 a of the magnetization free layer 6.A CoFeB alloy is employed as the interfacial magnetic layer 2 a of themagnetization reference layer 2, a FePt alloy is employed as theassisting magnetic layer 2 c, and a Ta film is employed as thecrystallization promoting layer 2 b.

In the TMR element having the specific example structure of Example 3, aCoFeB film is employed as the interfacial magnetic layer 6 a of themagnetization free layer 6, a FePt film having a L1₀ ordered structureis employed as the assisting magnetic layer 6 c, and a Ta film isemployed as the crystallization promoting layer 6 b. A Fe film isemployed as the interfacial magnetic layer 2 a of the magnetizationreference layer 2, a FePt film having a L1₀ ordered structure isemployed as the assisting magnetic layer 2 c, and a Mg film is employedas the crystallization promoting layer 2 b.

Here, examples of L1₀ alloy layers that can be used in the magnetizationfree layer include not only FePt alloys but also ferromagnetic alloyseach formed with an element X that is at least one element selected fromthe group consisting of Fe and Co, and an element Y that is at least oneelement selected from the group consisting of Pt and Pd. Typicalexamples are a FePt alloy of a L1₀ ordered structure, a FePd alloy of aL1₀ ordered structure, and a CoPt alloy of a L1₀ ordered structure. Toform a L1₀ ordered structure, the relative proportions of the element Xand the element Y should preferably indicate that the relativeproportion of the element X is in the range of 40 atomic % to 60 atomic%. Part of a magnetization free layer made of a XY alloy having theabove described L1₀ ordered structure may be replaced with Ni, Cu, Zn,or the like. In this manner, the saturation magnetization Ms can be madelower. In a case where part of the magnetization free layer is replacedwith Cu, Zn, or the like, the ordering temperature can be made lower.

Also, part of a magnetization free layer made of a XY alloy having theabove described L1₀ ordered structure may be replaced with Cu, Au, Ag,Ru, Rh, Ir, Os, or a rare earth element (Nd, Sm, Gd, Tb, or the like).

The above-described ferromagnetic XY alloy having the L1₀ orderedstructure is a FCT structure. By regulating the structure, largemagnetic crystalline anisotropy energy of approximately 1×10⁷ erg/cm³can be obtained in the [001] direction. In other words, excellentperpendicular magnetization characteristics can be achieved throughpreferential orientation toward the (001) plane. The saturationmagnetization is approximately in the range of 600 emu/cm³ to 1100emu/cm³. In a case where one of the above mentioned elements is added,saturation magnetization can be made lower through optimization, whileeffective magnetic crystalline anisotropy is maintained.

In the TMR elements of Examples 1 and 2 shown in FIGS. 5 and 6, thenoble metal such as Pt or Pd might diffuse into the interfacial magneticlayer 2 a having an amorphous structure, if the interfacial magneticlayer 2 a is formed on the assisting magnetic layer 2 c of themagnetization reference layer 2. As a result, crystallization of theinterfacial magnetic layer 2 a from an amorphous structure phase mightbe hindered. Therefore, the crystallization promoting layer 2 b isinserted between the interfacial magnetic layer film 2 a and theassisting magnetic layer 2 c, so that the noble metal such as Pt or Pdcan be prevented from diffusing into the interfacial magnetic layer 2 a.

The crystallization promoting layer 2 b serves to prevent diffusion. Inview of this, if the magnetization reference layer 2 has the assistingmagnetic layer 2 c of hard magnetism at the bottom as shown in FIGS. 5and 6, it is preferable to insert a crystallization promoting layer 2 bthat is not made of an element that is not solid-soluble in noble metalssuch as Pt and Pd. Other than Ta, the crystallization promoting layer 2b may be made of Mg, Ca, Sc, Ti, Sr, Y, Zr, Nb, Mo, Ba, La, Hf, W, orthe like.

The material CrFeB of the interfacial magnetic layer 2 a tends to havein-plane magnetization. Therefore, magnetic exchange coupling beingdisrupted by the insertion of the crystallization promoting layer 2 b isnot preferable to maintain the perpendicular magnetizationcharacteristics of the interfacial magnetic layer 2 a. In view of this,it is preferable to insert a rare earth element such as Ce, Pr, Nd, Sm,Eu, Tb, Dy, Ho, Er, Tm, Yb, or Lu. This is because magnetism can begenerated by the mixing at the time of film formation. Especially, Gd isa ferromagnetic material, even though it is a single element. Tomaintain the perpendicular magnetization, it is preferable that the filmthickness of the above described crystallization promoting layer is 1 nmor smaller. In the case of Gd, the film thickness should preferably be 2nm or less. However, to maintain the perpendicular magnetization of theinterfacial magnetic layer of the magnetization reference layer, theproduct of the saturation magnetization Ms and the film thickness tshould preferably be 4.0 [nm·T (nanometer·tesla)] or less.

Examples 4 and 5

Typical example of stacked structures that are TMR elements ofbottom-reference types in which Gd is employed for the crystallizationpromoting layers are now shown as Examples 4 and 5.

Example 4

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (1 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/crystallization promoting layer 2 b made of Gd(0.5 nm)/assisting magnetic layer 2 c made of FePt (10 nm)/under layer12/thermally oxidized Si substrate

In a TMR element of the bottom-reference type such as the TMR elementsof Examples 1 and 2, an alloy film formed with a rare earth element andan element X that is at least one element selected from the groupconsisting of Fe and Co may be employed as the crystallization promotinglayer. An alloy film formed with a rare earth element and the element Xhas perpendicular magnetization. Example 5 is a stacked structure in aTMR element of a typical bottom-reference type.

Example 5

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (1 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/crystallization promoting layer 2 b made ofCoFeTb (5 nm)/assisting magnetic layer 2 c made of FePt (10 nm)/underlayer 12/thermally oxidized Si substrate

Although the film thickness of the above CoFeTb film 2 b is 5 nm, it maybe in the range of 0.1 nm to 10 nm by optimizing the film formationprocess. Here, the relative proportion of Tb in the CoFeTb film is 50vol % or less in volume percent. The relative proportion of the rareearth element should preferably be 50 vol % or less. If a larger amountthan that is added, the exchange coupling between the interfacialmagnetic layer 2 a and the assisting magnetic layer 2 c becomes weaker,and the perpendicular magnetization might not be maintained.

Example 6

FIG. 8 shows a stacked structure of a TMR element as Example 6, which isa structure of a bottom-reference type in which the magnetization of themagnetization reference layer 2 is fixed by an antiferromagnetic layer7. The TMR element of this example shown in FIG. 8 is the same as theTMR element of Example 1 shown in FIG. 5, except that the assistingmagnetic layer 2 c of the magnetization reference layer 2 is replacedwith a magnetization pinned film 2 d having magnetization fixed by theantiferromagnetic layer 7. The materials used in this example are thosementioned in Examples 1 to 5. The magnetization directions of themagnetization pinned film 2 d and the interfacial magnetic layer 2 a maybe perpendicular to the film plane or may be parallel to the film plane.

Example 7

FIG. 9 is a cross-sectional view of a TMR element as Example 7 inaccordance with the present invention. The TMR element of this exampleis the same as the TMR element of Example 6, except that themagnetization pinned film 2 d that is a single-layer magnetic film isreplaced with a magnetization pinned layer 3 having a syntheticstructure. More specifically, the magnetization pinned layer 3 is astacked structure having a nonmagnetic film 3 b provided between amagnetic film (an interfacial magnetic layer) 3 a and a magnetic film 3c. The magnetic film 3 a and the magnetic film 3 c areantiferromagnetically coupled to each other via the nonmagnetic film 3b. The magnetization of the magnetization pinned layer 3 is fixed by theantiferromagnetic layer 7. The magnetization directions of themagnetization pinned layer 3 and the interfacial magnetic layer 2 a maybe perpendicular to the film plane or may be parallel to the film plane.

In practice, the antiferromagnetic layer 7 may be a FeMn alloy layer, aPtMn alloy layer, an IrMn alloy layer, a NiMn alloy layer, a PdMn alloylayer, a RhMn alloy layer, a PtCr alloy layer, a PtCrMn alloy layer, orthe like. The optimum film thickness is in the range of 5 nm to 20 nm.

In the synthetic structure, the nonmagnetic film 3 is inserted betweenthe interfacial magnetic layer 3 a and the magnetic film 3 c. Thenonmagnetic film 3 b may be made of Ru, Os, or Ir, and its optimum filmthickness is in the range of 0.5 nm to 3 nm. The synthetic structureutilizes interlayer coupling, and a film thickness with which theantiferromagnetic coupling becomes the strongest is used. In thesynthetic structure, the magnetization directions of the interfacialmagnetic layer 3 a and the magnetic film 3 c are antiparallel to eachother.

In Example 7 shown in FIG. 9, the crystallization promoting layer 2 b isinserted to an interfacial magnetic layer, and divides the interfacialmagnetic layer into the interfacial magnetic layer 2 a and theinterfacial magnetic layer 3 a. The film thickness of the interfacialmagnetic layer 2 a that is located near the barrier layer 4 needs to be1 nm or greater, with the influence of the mixing of the layers aboveand below the interfacial magnetic layer 2 a being taken intoconsideration. The magnetization directions of the interfacial magneticlayer 2 a and the interfacial magnetic layer 3 a are parallel to eachother.

Typical examples of TMR elements that are used with in-planemagnetization are now described as stacked structures of Examples 8 to12.

Example 8

This example is a specific example structure of Example 6 shown in FIG.8, and has the following stacked structure:

Cap layer 14/magnetization free layer 6 made of CoFeB (3nm)/intermediate layer (barrier layer) 4 made of MgO (1 nm)/interfacialmagnetic layer 2 a made of CoFeB (3 nm)/crystallization promoting layer2 b made of Ta (0.2 nm)/magnetization pinned film 2 d made of CoFe (2.5nm)/antiferromagnetic layer 7 made of MnPt (10 nm)/under layer12/thermally oxidized Si substrate

Example 9

This example is a specific example structure of Example 7 shown in FIG.9, and has the following stacked structure:

Cap layer 14/magnetization free layer 6 made of CoFeB (3nm)/intermediate layer (barrier layer) 4 made of MgO (1 nm)/interfacialmagnetic layer 2 a made of CoFeB (1.5 nm)/crystallization promotinglayer 2 b made of Ta (0.2 nm)/interfacial magnetic layer 3 a made ofCoFeB (1.5 nm)/nonmagnetic film 3 b made of Ru (0.85 nm)/magnetic film 3c made of CoFe (2.5 nm)/antiferromagnetic layer 7 made of PtMn (10nm)/under layer 12/thermally oxidized Si substrate

Typical examples of TMR elements that have perpendicular magnetizationare now described as stacked structures of Examples 10 to 12.

Example 10

This example is a specific example structure of Example 6 shown in FIG.8, and has the following stacked structure:

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (1 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/crystallization promoting layer 2 b made of Ta(0.2 nm)/magnetization pinned film 2 d made of FePt (10nm)/antiferromagnetic layer 7 made of FeMn (10 nm)/under layer12/thermally oxidized Si substrate

Example 11

This example is another specific example structure of Example 6 shown inFIG. 8, and has the following stacked structure in which a 5-nm thickCoFeTb film is used as the crystallization promoting layer 2 b:

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (1 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/crystallization promoting layer 2 b made ofCoFeTb (5 nm)/magnetization pinned film 2 d made of FePt (10nm)/antiferromagnetic layer 7 made of FeMn (10 nm)/under layer12/thermally oxidized Si substrate

In this example, the magnetization directions of the CoFeB layer of theinterfacial magnetic layer 2 a and the FePt layer of the magnetizationpinned film 2 d can be made antiparallel to each other by adjusting theCoFeTb composition. If the relative proportion of the rare earth elementTb exceeds the compensation point, the magnetization directions of theinterfacial magnetic layer 2 a and the magnetization pinned film 2 dbecome antiparallel to each other.

Example 12

This example is the same as Example 6 shown in FIG. 8, except that theinterfacial magnetic layer 2 a is replaced with a synthetic structureconsisting of a first magnetic film, a nonmagnetic film, and a secondmagnetic film. The structure of this example is as follows:

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (1 nm)/first magnetic film made ofCoFeB (1 nm)/nonmagnetic film made of Ru (0.8 nm)/second magnetic filmmade of CoFeB (1 nm)/crystallization promoting layer 2 b made of Ta (0.2nm)/magnetization pinned film 2 d made of FePt (10 nm)/antiferromagneticlayer 7 made of FeMn (10 nm)/under layer 12/thermally oxidized Sisubstrate

Next, TMR elements of Example 13 and Comparative Example formed by asputtering technique are described. For each of those TMR elements, thearea resistance RA and the TMR ratio were measured by an in-planeenergizing technique.

Example 13

The TMR element of this example is a specific example of the TMR elementof Example 1 shown in FIG. 5, and has the following stacked structure:

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (2 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/crystallization promoting layer 2 b made of Ta(0.2 nm)/assisting magnetic layer 2 c made of FePt (10 nm)/under layer12/thermally oxidized Si substrate

Comparative Example

This comparative example is the same as Example 13, except that thecrystallization promoting layer 2 b is not formed. The structure of thiscomparative example is as follows:

Cap layer 14/magnetization free layer 6 made of FePt (3 nm)/intermediatelayer (barrier layer) 4 made of MgO (2 nm)/interfacial magnetic layer 2a made of CoFeB (2 nm)/assisting magnetic layer 2 c made of FePt (10nm)/under layer 12/thermally oxidized Si substrate

The results of the measurement show that the RA in Comparative Exampleis approximately 20 kΩμm², and the RA in Example 13 is approximately 10kΩμm². Meanwhile, the TMR ratio does not become lower, and is maintainedat a constant value.

A TMR element of Example 14 formed by the same sputtering technique asabove is now described.

Example 14

The TMR element of this example is the same as the TMR element ofExample 13, except that the 0.2-nm thick crystallization promoting layer2 b made of Ta is replaced with a 10-nm thick CoFeTb film. This TMRelement has the following stacked structure:

Cap layer 14/FePt (3 nm)/intermediate layer (barrier layer) 4 made ofMgO (2 nm)/interfacial magnetic layer 2 a made of CoFeB (2nm)/crystallization promoting layer 2 b made of CoFeTb (10 nm)/assistingmagnetic layer 2 c made of FePt (10 nm)/under layer 12/thermallyoxidized Si substrate

Through transmission electron microscopic (TEM) observation, theinterfacial magnetic layer 2 a of Example 14 was observed. As a resultof cross-sectional TEM observation, crystallization in the CoFeB film ofthe interfacial magnetic layer 2 a was confirmed in Example 14. InComparative Example, however, the entire CoFeB film as the interfacialmagnetic layer 2 a was substantially an amorphous structure.

Further, the RA was measured by the in-plane energizing technique. Theresults showed that the RA in Comparative Example was approximately 20kΩμm⁻², and the RA in Example 14 having the CoFeTb film inserted as thecrystallization promoting layer 2 b was lowered to 1 kΩμm⁻².

As described above, in accordance with this embodiment, a low-resistanceTMR element can be obtained, and a magnetization reversal of themagnetization free layer can be caused with a low current. Thus, alow-resistance magnetoresistive element of a spin-injection write typecan be provided.

Second Embodiment

Next, a MRAM of a spin injection write type in accordance with a secondembodiment of the present invention is described.

The MRAM of this embodiment includes memory cells. FIG. 10 is across-sectional view of one of the memory cells of the MRAM of thisembodiment. As shown in FIG. 10, the upper face of an MR element 1 isconnected to a bit line 32 via an upper electrode 31. The lower face ofthe MR element 1 is connected to a drain region 37 a of the source anddrain regions on the surface of a semiconductor substrate 36 via a lowerelectrode 33, an extension electrode 34, and a plug 35. The drain region37 a, a source region 37 b, a gate insulating film 38 formed on thesubstrate 36, and a gate electrode 39 formed on the gate insulating film38 constitute a selective transistor Tr. The selective transistor Tr andthe MR element 1 form the one memory cell of the MRAM. The source region37 b is connected to another bit line 42 via a plug 41. Alternatively,the plug 35 may be provided under the lower electrode 33 without theextension electrode 34, and the lower electrode 33 may be connecteddirectly to the plug 35. The bit lines 32 and 42, the electrodes 31 and33, the extension electrode 34, and the plugs 35 and 41 are made of W,Al, AlCu, Cu, and the likes.

In the MRAM of this embodiment, memory cells each having the samestructure as the memory cell shown in FIG. 10 are arranged in a matrixform, so as to form the memory cell array of the MRAM. FIG. 11 is acircuit diagram showing the principal components of the MRAM of thisembodiment.

As shown in FIG. 11, memory cells 53 that are formed with MR elements 1and selective transistors Tr are arranged in a matrix form. One end ofeach of the memory cells 53 arranged in the same column is connected tothe same bit line 32, and the other end is connected to the same bitline 42. The gate electrodes (word lines) 39 of the memory cells 53arranged in the same row are connected to one another, and are alsoconnected to a row decoder 51.

The bit line 32 is connected to a current source/sink circuit 55 via aswitch circuit 54 such as a transistor. The bit line 42 is connected toa current source/sink circuit 57 via a switch circuit 56 such as atransistor. The current source/sink circuits 55 and 57 supply writecurrent (inversion current) to the connected bit lines 32 and 42, andremove the write current from the connected bit lines 32 and 42.

The bit line 42 is also connected to a read circuit 52. The read circuit52 may be connected to the bit line 32. The read circuit 52 includes aread current circuit, a sense amplifier, and the likes.

At the time of writing, the switch circuits 54 and 56 connected to thememory cell on which writing is to be performed, and the selectivetransistor Tr are turned on, so as to form a current path that runsthrough the subject memory cell. One of the current source/sink circuits55 and 57 functions as a current source, and the other one functions asa current sink, in accordance with the information to be written. As aresult, the write current flows in the direction determined by theinformation to be written.

As for the write speed, it is possible to perform spin-injection writingwith a current having a pulse width of several nanoseconds to severalmicroseconds.

At the time of reading, a read current of such a small size as not tocause a magnetization reversal is supplied to the subject MR element 1by a read current circuit in the same manner as in the case of writing.The read circuit 52 compares the current value or the voltage valuedetermined by the resistance value in accordance with the magnetizationstate of the MR element 1, with a reference value. In this manner, theread circuit 52 decides the resistive state.

At the time of reading, the current pulse width should preferably besmaller than the current pulse width observed in a writing operation.Accordingly, write errors with the current at the time of reading can bereduced. This is based on the fact that the absolute value of the writecurrent is larger when the pulse width of the write current is smaller.

As described so far, in accordance with each embodiment of the presentinvention, low-resistance TMR elements are used as memory elements.Accordingly, the magnetization free layer can be caused to have amagnetization reversal with a low current. Thus, a low-resistancemagnetoresistive random access memory of a spin-injection write type canbe provided.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

1. A magnetoresistive element comprising: a magnetization referencelayer having magnetization perpendicular to a film plane, a direction ofthe magnetization being invariable in one direction; a magnetizationfree layer having magnetization perpendicular to the film plane, adirection of the magnetization being variable; and an intermediate layerprovided between the magnetization reference layer and the magnetizationfree layer, at least one of the magnetization reference layer and themagnetization free layer including: an interfacial magnetic layer formedin contact with the intermediate layer, and having a crystalline phasecrystallized from an amorphous structure; and a crystallizationpromoting layer formed in contact with the interfacial magnetic layer onthe opposite side from the intermediate layer, and promotingcrystallization of the interfacial magnetic layer, the magnetizationdirection of the magnetization free layer being variable by flowing acurrent between the magnetization reference layer and the magnetizationfree layer via the intermediate layer.